The present invention relates to an exhaust system for a lean burn internal combustion engine, and in one illustrative embodiment, to an exhaust system for a diesel engine.
Lean burn internal combustion engines, such as diesel engines and gasoline engines, produce a number of pollutants including carbon monoxide (CO), unburnt hydrocarbon (HC), particulate matter (PM) and nitrogen oxides (NOx). Whilst not as visible to the naked eye as PM emitted by diesel engines, gasoline engines produce PM of the size-order of <1 μm such as 10-100 nm. Interest in gasoline PM is growing as particles of this size can penetrate deep into the human lungs and can be detrimental to health.
The amount of these pollutants that can be emitted by vehicular internal combustion engines is prescribed by legislation in various countries and regions of the world, such as the USA and Europe, and these amounts are set to decrease as the legislation tightens step-wise over the next ten years or so. Similarly, International agreements between countries have led to moves toward vehicular internal combustion engines that use fuel more efficiently. The legislation acts as a stimulus to vehicle manufacturers and to their suppliers to devise new engines that are more fuel-efficient and that emit fewer pollutants and to exhaust systems that can clean up the exhaust gas before it passes to atmosphere.
One such exhaust system component primarily for treating diesel exhaust comprises an oxidation catalyst for oxidising NO in the exhaust gas to NO2 and a downstream filter for trapping PM. A process for treating diesel PM that uses this arrangement is described in EP-B-0341382 or U.S. Pat. No. 4,902,487, both of which are incorporated herein by reference. The process claimed in EP-B-0341382 comprises passing an exhaust gas, such as a diesel exhaust gas, including PM and NO unfiltered over an oxidation catalyst to convert the NO to NO2, collecting soot on the filter and combusting the collected soot by reaction with the NO2. This technology is commercially available as Johnson Matthey's Continuously Regenerating Trap or CRT®. Further steps may be added, for example downstream NOx removal by injection of reductant e.g. HC or NOx-specific reactant e.g. NH3 or urea (see for example our WO-A-00/21647, incorporated herein by reference). An advantage of this process is that it is possible to combust diesel soot at temperatures of up to 400° C., whereas combustion of diesel soot in oxygen occurs at about 500° C. This is significant since diesel exhaust gas is generally cooler than exhaust gas from gasoline engines and soot would accumulate on the filter causing back-pressure problems in the system if the process relied on combustion of soot in oxygen.
One form of gasoline engine is a gasoline direct injection engine, which is designed to operate under stoichiometric and lean conditions. When running lean, relatively low levels of NOx are formed that cannot be reduced (removed) in the presence of the relatively high levels of oxygen in the exhaust gas. Reducing species, e.g. HC, can reduce NOx to N2 during stoichiometric- or rich-running conditions, as comparatively less oxygen is present than during lean-running conditions.
In order to control NOx in lean-bum engines, there has been devised a NOx absorber/catalyst which can store NOx, e.g. as nitrate, when an engine is running lean. In a stoichiometric or rich environment, the nitrate is understood to be thermodynamically unstable, and the stored NOx is released and is reduced by the reducing species present in the exhaust gas. This NOx absorber/catalyst is commonly called a NOx-trap and is described in EP-A-0560991 (the entire contents of which is incorporated herein by reference). By periodically controlling the engine to run stoichiometrically or rich, stored NOx is reduced and the NOx-trap regenerated.
A typical NOx-trap formulation comprises a washcoat comprising a catalytic oxidation component, such as platinum, a NOx-storage component, such as barium, and a reduction catalyst e.g. rhodium on a surface area-enhancing support material, e.g. a metal oxide such as alumina, or a mixed or composite (i.e. not a mixed phase pure) metal oxide containing one or more of aluminium, zirconium, titanium, cerium, silicon and chromium. One mechanism commonly given for NOx-storage during lean engine operation for this formulation is:NO+½O2→NO2; and  (i)BaO+NO2+½O2→Ba(NO3)2 .  (ii)
In the first step, the nitric oxide reacts with oxygen on active oxidation sites on the platinum to form NO2. The second step involves adsorption of the NO2 by the storage material in the form of an inorganic nitrate.
When the engine runs under rich conditions or at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO or NO2 according to equation (iii) below. Under rich conditions, these nitrogen oxides are subsequently reduced by carbon monoxide, hydrogen and hydrocarbons to N2, which can take place over the reduction catalyst.Ba(NO3)2→BaO+2NO+ 3/2O2 or Ba(NO3)2→BaO+2NO2+½O2; and   (iii)NO+CO→½N2+CO2 (and other reactions).  (iv)
In the reactions of (i)-(iv) above the reactive barium species is given as the oxide. However, it is understood that in the presence of air most of the barium is in the form of the carbonate or possibly the hydroxide. The above reaction schemes can be adapted accordingly for species of barium other than the oxide.
NOx-traps are typically coated on a flow through honeycomb monolith substrates. One form of such substrate is made from a ceramic material such as cordierite. Typical commercially available flow-through monolith substrates have a mean pore size of 3 μm and a porosity of about 35%. NOx-trap washcoat formulations for coating on flow through honeycomb monolith substrates have a D50, of about 15 μm, i.e. at least 50% of the particle sizes in the washcoat are 15 μm in diameter or less. Similarly a D70, would mean that at least 70% of the particle sizes in the washcoat are 15 μm in diameter or less.
Using sophisticated engine management techniques to provide for rich/lean cycling and common rail fuel injection, vehicle manufacturers are now adopting NOx-trap technology into diesel exhaust systems. One such system is described in EP-A-0758713 (the entire contents of which is incorporated herein by reference). Means for reducing the redox composition of the exhaust gas, as defined by lambda, for the purpose of regenerating a NOx-trap include injecting HC into the exhaust gas downstream of the engine, adjusting the ignition timing of at least one engine cylinder or adjusting the engine air-to-fuel ratio. Since the NOx trap disclosed in EP-A-0758713 comprises a flow through monolith substrate, it can be assumed that the substrate porosity and mean pore size of the substrate and the D50 of the washcoat composition will be in the order of that mentioned above.
Where a NOx-trap formulation comprises a reduction catalyst such as rhodium, it is known to locate the NOx-trap components where they can maximise their activity, i.e. of the oxidation catalyst component, e.g. platinum, to oxidise NO to NO2 and HC and CO during lambda >1 conditions; and for the rhodium to catalyse the reduction of NOx to N2 with HC during 1 ≧lambda conditions. One such arrangement is disclosed in U.S. Pat. No. 6,413,483 (the entire contents of which is incorporated herein by reference), wherein the rhodium component is in an overlayer on a layer containing the Pt component.
Another technique that can be used to control emissions is exhaust gas recirculation (EGR). In this, a portion of the exhaust gas is taken returned to the engine air intake so that the engine is fed a mixture of air and exhaust gas. Because the resulting mixture is lower in oxygen than in air, the temperature of the combustion event is reduced so that there is less NOx in the exhaust gas. This technique does cause an increase in PM, so there is a pay-off between NOx and PM, but by managing the rate of EGR to the load on the engine, it is possible to obtain an overall reduction in pollutant emissions.
In Japanese Patent No. 2722987 and EP-A-1079084 (the entire contents of which documents are incorporated herein by reference), Toyota describes an exhaust system including a component including a combination of certain of the catalyst features described above. Essentially it describes a catalysed particulate trap including a NOx-trap. In particular, the component comprises a particulate trapping device comprising a NOx absorbent capable of absorbing NOx contained in exhaust gas when the air-fuel ratio of the exhaust gas is lean, and capable of releasing the NOx absorbed in the NOx component when the air-fuel ratio of the exhaust gas is substantially equal to the stoichiometric air-fuel ratio or rich. One method of coating a particulate filter with a catalyst washcoat is disclosed in EP-A-0766993 (the entire contents of which is incorporated herein by reference). The method of this patent provides a coating layer which is uniformly formed on the surfaces of the pores in the porous walls of the wall-flow filter and the catalyst is carried thereon.
In JP-B-2722987, the mechanism suggested for the combustion of soot trapped on the particulate trap is that during lean running, a high concentration of oxygen O2 is deposited in the form of O2− or O2−on the surface of platinum (Pt). NO contained in the flowing exhaust gas reacts with O2− or O2−on the surface of the Pt to form NO2 (2NO+O2→2NO2). Then, part of the NO2 thus formed is absorbed into the NOx absorbent while being oxidised on Pt, and diffused in the form of nitrate ion NO3− while combining with BaO.
If the air-fuel ratio is adjusted rich, the oxygen concentration in the exhaust gas is reduced, and consequently the amount of NO2 formed on the surface of the Pt is reduced. If the amount of NO2 produced is reduced, the reaction proceeds in the reverse direction (NO3−→NO2) and thus the nitrate ion NO3− is released in the form of NO2 from the absorbent.
The suggestion is that “activated oxygen” species such as O2− and O2−are responsible for combusting particulate during rich and lean running, but also that NO2 could also be responsible for combustion of particulate, particularly during rich running.
We have investigated Toyota's combined particulate filter-NOx trap and have found, very surprisingly, that by introducing an oxidation catalyst active for oxidation of NO to NO2 upstream of the filter/trap in a similar arrangement to that described in EP-B-0341382 or U.S. Pat. No. 4,902,487 that filter regeneration is improved compared with filter regeneration employing the particulate filter-NOx trap alone. We have been able to show this by measuring the back-pressure in the system on a bench mounted engine. Increased back-pressure is an indication of increased particulate build up, i.e. that particulate deposition and particulate combustion are not in balance. It is also believed that the system represents an improvement over the system described in EP-A-758713 in that NOx released from the NOx absorbent can combust trapped particulate, but also oxidise HC to carbon dioxide (CO2) and water (H2O) and oxidise carbon monoxide CO to CO2. Accordingly, the system provides an improved management of pollutant species in the exhaust gas.